The invention relates to a device and a method for detecting the position of an object, in particular, the armature of a valve, for example, an inlet and outlet valve, fuel injection valve, gas exchange valve, or the like, utilizing at least two coils, preferably two magnet coils, which can be energized for moving the object between the two coils.
Devices and methods for detecting the position of an object have been known from practice for a long time, for example, from DE 197 35 375 C1, and corresponding U.S. Pat. No. 6,016,778, which disclose a device for detecting the position of an object, namely an armature of a valve. The known device comprises a piezoelectric element for detecting the position of the armature of the valve, with the valve comprising two magnet coils, which can be energized for moving the object between the two coils. The detection of the position of the armature occurs indirectly by way of measuring the spring force between two springs, which hold the armature in a center position between two end positions, when the coils are not energized. By means of the piezoelectric element, it is thus possible to determine based on the spring force, the speed of the armature for adjusting with the signal generated by the piezoelectric element the circuit for controlling the magnet drive, so that a minimal impact speed of the armature is reached in its end position. Ideally, the springs have a linear characteristic, so that for this simplified case, the spring force changes in a linearly proportionate relationship with the position of the armature.
The known device is especially problematic in that the piezoelectric element permits detecting only the end positions of the armature. Other positions of the armature are computed indirectly via the characteristic of the springs. This leads to added errors in the detection of the position of the armature.
In addition, DE 198 56 528 A1 discloses, when viewed by itself, a valve lift sensor, which includes two stratified bodies. The stratified bodies are arranged at a distance with a space being formed therebetween. This space accommodates a Hall sensor. An object made as a sensor element is provided with a magnetic element and arranged for displacement relative to the Hall sensor. The Hall sensor operates in this case by the noncontacting method. The device disclosed in DE 198 56 528 A1 is especially problematic in that temperature changes on the Hall sensor are not compensated. In addition, the nonlinear output signal has a disadvantageous effect on the detection of the position of the object. Further disclosed are, when viewed alone, the transverse armature construction for sensors, as well as a detection by measuring current and inductance via the same coil.
It is therefore an object of the present invention to provide a device and a method for detecting the position of an object of the initially described type, wherein the position of the object can be detected with a simplest construction in a largely linear and troublefree way.
In accordance with the invention the foregoing object is accomplished by the device and method for detecting the position of an object wherein the two coils are alternately used for moving the object between the coils and for detecting the position of the object.
To begin with, it has been recognized by way of the present invention that additional means for detecting the position of an object, such as, for example, piezoelectric elements, are only inadequately suited for detecting the position of an object. By way of the present invention, it has furthermore been recognized that, departing from the practice of the past, i.e. the use of additional means, it is possible to accomplish a detection of the position of the object solely and alone by means of the two coils. In a technical respect, this is realized in a surprisingly simple manner, in that the two coils, which are normally used only and alone for moving the object between the coils, are now alternately used both for moving the object between the coils and for detecting the position of the object. This is accomplished, for example, in that while the one coil is used for moving the object, the other one is used for detecting the object, namely as a kind of eddy current sensor.
Within the scope of a particularly simple configuration, the object is arranged on a stem. This ensures a particularly uniform movement of the object, in particular when the object takes the form of an armature of a valve.
As regards a particularly robust and functional configuration, the coils could be arranged in at least one body. In a particularly advantageous manner, the body could be ferromagnetic.
As regards a particularly functional configuration, the object could be adapted for movement between two end positions. In this arrangement, the object could be held by means of at least one spring in a position, in particular an end position of the valve. In the case that the object is the armature of a valve, the position in which the object is held by means of the spring could very advantageously be the closing position of the valve. However, it would also be possible that the object is held by means of two springs in any position.
In a further advantageous manner, at least one coil could be subdivided into at least two sections. In this instance, the first section could be arranged closer to the object than the second section. In addition or as an alternative, the spacing between the second section and the object could be greater than half the diameter of the coil.
Preferably, the impedance of the first section could additionally be greater, in particular about three to five times greater than the impedance of the second section. In the case of such a configuration, the movement of the object would substantially influence only the impedance of the first section, with the impedance of the second section being largely independent of the position of the object.
Within the scope of a particularly simple configuration, the coil or coils could each include at least two contact points and at least one tap. In this connection, it would be possible to arrange the contact points and the tap at the end of the sections. This would especially simplify the energizing and the measuring of the impedance of the respective section.
In a particularly advantageous manner, the quality factor of the two sections of the coil could be the same, when the object is in an end position. In a particularly simple manner, the adaptation of the quality factor could be realized by adapting the ratio of the windings of the first and the second section.
With respect to a particularly simple circuit, at least one of the coils could form with an operational amplifier a voltage-current converter. In this instance, the first section of the coil could be supplied by the voltage-current converter, with the current being dependent on the impedance of the second section.
The tap between the first and the second section could connect to the inverting input of the operational amplifier. The noninverting input of the operational amplifier could connect in addition to a multiplexer. By means of the multiplexer, it would be possible to generate a voltage Uin, which could be, for example, a square-wave voltage.
In a particularly advantageous manner, the multiplexer could be controllable by means of a microcomputer or a quartz oscillator. The frequency of the microcomputer or quartz oscillator could be much higher, for example, 50 kHz to 250 kHz, than the frequency, at which the coil is activated for moving the object.
With respect to a particularly simple configuration, it would be possible to determine the voltage drop in the first section by means of an instrumental amplifier, with the relation being:       U    v    =            K      ·              U        in            ·        |                  Z        s1                    Z        s2              |          ·              e        γχ            
where Uin is the voltage at the noninverting input of the operational amplifier; Zs1 the impedance of the first section, when the object is in its end position, in which the spacing between the coil and the object is minimal; Zs2 the impedance of the second section, which is essentially independent of the distance of the object from the coil; K the amplification factor of the instrumental amplifier; and xcex3 a coefficient, which is dependent on the geometry of the coil. The voltage UV at the output of the instrumental amplifier is thus dependent on the ratio of the impedances Zs1 and Zs2 of the first and second sections.
The output of the instrumental amplifier could connect to the input of a differentiator. The output of the differentiator could then have the voltage:       U    s    =      K    ·          [                        U          in                ·                              Z            s1                                Z            s2                              ]        ·          e      γχ        ·          e                        -          t                /        τ            
where xcfx84 is the time constant of the differentiator.
The output of the differentiator could connect to a comparator. This would make it possible to compare the output voltage of the differentiator US by means of the comparator with a constant voltage UO. At the output of the comparator, it will thus be possible to generate a pulse width-modulated signal, when the multiplexer is controlled by means of the microcomputer or quartz oscillator.
As an alternative, the output of the comparator could connect to a multivibrator, in particular a monostable multivibrator. The output signal of the multivibrator could then be used for controlling the multiplexer. In this case, the device would operate in the way of a free-swinging oscillator with a period T=tx+xcex94t, where xcex94t is the time constant of the monostable multivibrator.
In a particularly simple manner, it would then be possible to determine the position of the object by means of an evaluation circuit. This evaluation circuit could be realized in any form.
In accordance with the invention, the foregoing object is also accomplished by the method wherein the two coils are alternately used for moving the object between the coils and for detecting the position of the object.
With respect to a particularly reliable detection of the position of the object, the coil used for the detection could be supplied with a high-frequency current, preferably of a small amplitude. The impedance of the coil used for detecting the object would then be exponentially dependent on the distance of the object. Thus, the coil used for the detection would operate by the eddy current principle. In this connection, it is possible to determine the position of the object linearly and at the same time in a temperature stable manner and independently of fluctuations of the supply voltage. A temperature compensation could be realized in addition in a different way, for example, by means of the arrangement of a compensation coil.
At least one coil could form with at least one operational amplifier a voltage-current converter.
Within the scope of a particularly functional development, at least one coil could be subdivided into at least two sections. In this case, the first section could be arranged closer to the object than a second section, so that the position of the object could be determined by changing the impedance of the first section. The operating principle is thus based on the effect of the demagnetization action of eddy currents which are induced in the object by the electromagnetic field of the coil. This results in that the impedance of the first section of the coil changes considerably with the movement of the object. At the same time, the impedance of the second section changes relatively little with respect to the location of the object, since it is arranged adequately removed from the object. The first section of the coil could be supplied by a source of current, namely the voltage-current converter that is formed by the coil and the operational amplifier. In this instance, the current would be dependent on the impedance of the second section.
The voltage drop in the first section could be determined by means of an instrumental amplifier, with the relation being:       U    s    =            U      o        =          K      ·              [                                            U              in                        ·                    |                                    Z              s1                                      Z              s2                                |                ]            ·              e        γχ            ·              e                              -                                          t                0                            ⁢                              (                x                )                                              /          τ                    
The output of the instrumental amplifier could then be connected to the input of a differentiator. At the output of the differentiator, the voltage would thus be:       U    s    =      K    ·          [                                    U            in                    ·                |                              Z            s1                                Z            s2                          |            ]        ·          e      γχ        ·          e                        -                                    t              0                        ⁢                          (              x              )                                      /        τ            
where xcfx84 is the time constant of the differentiator.
The output of the differentiator could then be connected to a comparator, so that the voltage at the output of the differentiator:       U    s    =            U      o        =          K      ·              [                                            U              in                        ·                    |                                    Z              s1                                      Z              s2                                |                ]            ·              e        γχ            ·              e                              -                                          t                0                            ⁢                              (                x                )                                              /          τ                    
is compared by means of the comparator with a constant voltage UO.
For certain positions of the object, only signals of the duration t0(x) are applied to the output of the comparator. Thus, the time is proportionate to the position of the object, namely:             t      0        ⁡          (      X      )        =            τ      ·      In      ·              [                              K            ·                                          U                in                                            U                o                                      ·                    |                                    Z              s1                                      Z              s2                                |                ]              +    γX  
By means of changing the constant voltage and/or the time constants, it would then be possible to adjust the values of the time.
There exist various possibilities of improving and further developing the teaching of the present invention in an advantageous manner. To this end, one may refer to the following detailed description of preferred embodiments of the device and the method in accordance with the invention for detecting the position of an object with reference to the drawing. In conjunction therewith, generally preferred improvements and further developments of the teaching are explained.